GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care — Presented by Campaign for Accountability — House passes drug pricing bills amid ObamaCare row | Senate Republicans running away from Alabama abortion law | Ocasio-Cortez confronts CEO over K drug price tag Bipartisan senators unveil measure to end surprise medical bills Work on surprise medical bills goes into overdrive MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonWarren policy ideas show signs of paying off Biden at campaign kickoff event: I don't have to be 'angry' to win Top Dem: Trump helps GOP erase enthusiasm gap; Ohio a big problem MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchTrump gambles in push for drug import proposal Biden's role in Anita Hill hearings defended by witness not allowed to testify 'Congress' worst tax idea ever'? Hardly. MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerCorker: 'I just don't' see path to challenge Trump in 2020 Ex-GOP Sen. Corker: Trump primary would be 'good thing for our country' Pollster says Trump unlikely to face 'significant' primary challenge MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellOn The Money: Treasury rejects Dem subpoena for Trump tax returns | Companies warn trade war about to hit consumers | Congress, White House to launch budget talks next week | Trump gets deal to lift steel tariffs on Mexico, Canada Schumer calls on McConnell to hold vote on Equality Act House Dem cites transgender grandson in voting for Equality Act MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerGetting serious about infrastructure Schumer calls on McConnell to hold vote on Equality Act 'SleepyCreepy Joe' and 'Crazy Bernie': Trump seeks to define 2020 Dems with insults MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayOvernight Health Care — Presented by Campaign for Accountability — House passes drug pricing bills amid ObamaCare row | Senate Republicans running away from Alabama abortion law | Ocasio-Cortez confronts CEO over K drug price tag Bipartisan senators unveil measure to end surprise medical bills Work on surprise medical bills goes into overdrive MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeyTrump faces criticism for hosting Hungary's leader Bill Nye tees off on climate change skeptics: 'The planet is on f---ing fire!' Sanders to join Ocasio-Cortez in headlining Green New Deal rally Monday MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenGOP rep introducing bill offering alternative Medicaid program The Hill's Morning Report - Barr stiff-arms House following Senate grilling Overnight Energy — Presented by Job Creators Network — House Republican tries to force Green New Deal vote | 'Awkward' hearing to vet Interior nominee and watchdog | House panel approves bill to stop drilling in Arctic refuge MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpTrump calls for Republicans to be 'united' on abortion Tlaib calls on Amash to join impeachment resolution Facebook temporarily suspended conservative commentator Candace Owens MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnGraham warns of 5G security threat from China Export-Import Bank back to full strength after Senate confirmations Trump, Senate GOP discuss effort to overhaul legal immigration MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.